Where an electron beam apparatus such as a scanning electron microscope automatically effects both focusing and astigmatism correction, an automatic focusing operation is first performed. In this focusing operation, the focus is brought into the position of the circle of least confusion. Then, an astigmatism correction is made. To bring the focus into the position of the circle of least confusion, an electrical current supplied to the objective lens or to a lens auxiliary to the objective lens is varied in a stepwise fashion. The position of the focus is shifted for each different value of the lens current so that the electron beam scans the surface of a specimen in two dimensions. During the two-dimensional scan, secondary electrons or reflected electrons emanating from the specimen are detected by a detector. The output signal from the detector is integrated for one frame of image. The signal distribution curve shown in FIG. 1 is obtained from the integrated values derived from every value of lens current. The integrated values are plotted on the vertical axis of FIG. 1. The intensity of excitation of the objective lens is plotted on the horizontal axis. It is known that the position of the peak of this distribution curve is the position of the circle of least confusion. The objective lens current is so controlled that the intensity of excitation of the lens corresponds to the position of the peak. Then, an automatic astigmatism correction is made. Subsequently, a second automatic focusing operation is performed. FIG. 2 is a flowchart illustrating these focusing operations and astigmatism correction operations.
Where the electron beam involves astigmatism, the above-described signal distribution curve has two peaks as shown in FIG. 3. In this case, it is known that the position of least confusion lies midway between the two peaks P1 and P2. If the difference in height between the two peaks is great, then there is the possibility that the control system erroneously regards the maximum peak position P1 as the position of least confusion. At this time, if the focus is brought to an overlying position that is not the position of least confusion, and if an astigmatism correction is made at that position, then the correction is not carried out accurately.